Conventionally, power supply devices for air conditioners, refrigerators and the like receive alternate current (ac) voltage from commercial power supply, rectify the ac voltage by a rectifier circuit such as a diode bridge, smooth the rectified voltage by a smoothing circuit such as a capacitor to generate direct current (dc) voltage, convert the dc voltage by an inverter to ac voltage, and supply the ac voltage to an ac motor.
Such a power supply device's power factor is improved and its power supply harmonic current is reduced, by a method introducing an active filter between a rectifier circuit and a smoothing circuit to match an input current and an input voltage in waveform and phase, as described in Japanese Patent Laying-Open Nos. 8-33392 (Patent Document 1) and 8-182329 (Patent Document 2) for example.
Furthermore, when a power supply device employing an active filter is used, and low ac voltage is received, a large electric power loss is caused. This is resolved by a method allowing the active filter to receive and output voltages, respectively, with a constant difference therebetween, as described in Japanese Patent Laying-Open No. 2006-20402 (Patent Document 3) for example.
Furthermore, a power supply device employing an active filter has a switching element switched at a high frequency, and as the switching element is increased in temperature, it has a high risk of destruction. Accordingly, the switching element's temperature is detected and if it is high a target voltage is reduced, as described in Japanese Patent Laying-Open No. 9-70178 (Patent Document 4) for example.